Complementary metal oxide semiconductor (CMOS) image sensor arrays often have pixel well capacities as small as 10,000 (or less) electrons for full exposure and, when incorporated into a still image camera employing a global shutter, are required to retain charge on their hold capacitors during exposure and/or the readout time of the entire array. Resulting pixel well leakage currents on the order of one fA can unacceptably reduce the signal-to-noise ratio in such sensor arrays. Pixel leakage currents can be attributed to a variety of sources including gate current, diffusion current, device non-zero off-current and diode dark current. Moreover, pixel leakage current includes both static and dynamic components.
Conventional arrays have compensated for static leakage currents by adding a single, unexposed, extra pixel (often called a “dark pixel”) in each row of the array. To compensate for static leakage current, the integrated current due to light-generated electron-hole pairs can be normalized against the integrated static charge developed on that row's dark pixel. Dynamic leakage currents, on the other hand, are highly dependent upon the signal amplitude at a given pixel's sample/hold node. These dynamic leakage currents often cause read-out error terms in excess of the static leakage induced error and may not be adequately compensated for using conventional dark pixel techniques.